In the default scene with the 'Cube' geometry selected. Goto 3D View's Menu and change to 'Edit' Mode​

Goto 'Tools' from the Tool Shelf. Under 'Add:', select 'Subdivide'

Under 'Subdivide'. Input '3' under Number of Cuts and '1' for Smoothness

Switch to 'Right Ortho' View (no. '3' and '5' from the Number pad on keyboard)Hit keyboard 'S' (Scale), then 'Y' (Y-axis constraint) and input '0.5' to scale it down.

Alike the image below. Switch to 'Top Ortho' view (no. '7' from Number pad). In 'Edit' Mode, Click 'Face Select' and multi-select the top four faces on the right hand side (hold 'Shift' key to multiselect faces.

In the 'Tool Self', click '3D Printing' tab and firstly select the geometry object, then under 'Checks:', click 'Check All' to see result.*make sure there is no 'Non Manifold Edge' or fix before exporting to .stl